In communication systems, a goal is to transport information from one physical location to another. It is typically desirable that the transport of this information is reliable, is fast and consumes a minimal amount of resources. Methods of information transport are broadly categorized into “baseband” methods that dedicate use of the physical communications channel to one transport method, and “broadband” methods that partition the physical communications channel in the frequency domain, creating two or more independent frequency channels upon which a transport method may be applied.
Baseband methods may be further categorized by physical medium. One common information transfer medium is the serial communications link, which may be based on a single wire circuit relative to ground or other common reference, multiple such circuits relative to ground or other common reference, or multiple such circuits used in relation to each other. A common example of the latter uses differential signaling (“DS”). Differential signaling operates by sending a signal on one wire and the opposite of that signal on a matching wire. The signal information is represented by the difference between the wires, rather than their absolute values relative to ground or other fixed reference.
Parallel data transfer is also commonly used to provide increased interconnection bandwidth, with busses growing from 16 or fewer wires, to 32, 64, and more. As crosstalk and noise induced on the parallel signal lines can produce receive errors, parity was added to improve error detection, and signal anomalies were addressed through active bus termination methods. However, these wide data transfer widths inevitably resulted in data skew, which became the limiting factor in increased bus data transfer throughput. Alternative approaches were developed utilizing narrower bus widths operating at much higher clock speeds, with significant effort placed on optimizing the transmission line characteristics of the interconnection medium, including use of impedance-controlled connectors and micro stripline wiring. Even so, the inevitable path imperfections required use of active equalization and inter-symbol interference (ISI) elimination techniques, including active pre-emphasis compensation for transmitters and Continuous Time Linear Equalization (CTLE) and Decision Feedback Equalization (DFE) for receivers, all of which increased the complexity and power consumption of the communications interface.
A number of signaling methods are known that maintain the desirable properties of DS, while increasing pin efficiency over DS. One such method is Vector signaling. With vector signaling, a plurality of signals on a plurality of wires is considered collectively although each of the plurality of signals might be independent. Thus, vector signaling codes can combine the robustness of single circuit DS and the high wire count data transfer throughput of parallel data transfer. Each of the collective signals in the transport medium carrying a vector signaling codeword is referred to as a component, and the number of plurality of wires is referred to as the “dimension” of the codeword (sometimes also called a “vector”). With binary vector signaling, each component or “symbol” of the vector takes on one of two possible values. With non-binary vector signaling, each symbol has a value that is a selection from a set of more than two possible values. The set of values that a symbol of the vector may take on is called the “alphabet” of the vector signaling code. A vector signaling code, as described herein, is a collection C of vectors of the same length N, called codewords. Any suitable subset of a vector signaling code denotes a “subcode” of that code. Such a subcode may itself be a vector signaling code. In operation, the coordinates of the codewords are bounded, and we choose to represent them by real numbers between −1 and 1. The ratio between the binary logarithm of the size of C and the length N is called the pin-efficiency of the vector signaling code. A vector signaling code is called “balanced” if for all its codewords the sum of the coordinates is always zero. Additional examples of vector signaling methods are described in Cronie I, Cronie II, Cronie III, Cronie IV, Fox I, Fox II, Fox III, Holden I, Shokrollahi I, Shokrollahi II, and Hormati I.
As previously described, broadband signaling methods partition the available information transfer medium in the frequency domain, creating two or more frequency-domain “channels” which may then may transport information in a comparable manner to baseband circuits, using known methods of carrier modulation to convert the baseband information into a frequency-domain channel signal. As each such channel can be independently controlled as to amplitude, modulation, and information encoding, it is possible to adapt the collection of channels to widely varying information transfer medium characteristics, including variations in signal loss, distortion, and noise over time and frequency.
Asymmetric Digital Subscriber Line or ADSL is one widely deployed broadband signaling method used to transport digital data over legacy copper telephony circuits. In ADSL, each of potentially several hundred frequency-domain channels is independently configured for amplitude, modulation method, and digital carrying capacity, based on the particular noise and loss characteristics of the copper circuit being used for transport.